A magnetic tunnel junction (MTJ) is composed of two ferromagnetic material layers and a tunnel barrier layer (a tunnel insulating layer) put between these ferromagnetic material layers, and the resistance changes largely, depending on the relative directions of the magnetizations of the ferromagnetic material layers. Such a phenomenon is called a tunnel magneto-resistance effect (TMR effect). By detecting the resistance of the magnetic tunnel junction, the direction of the magnetization of the ferromagnetic material layer can be determined. A magnetic random access memory (MRAM) which can hold data in a non-volatile manner is known as a magneto-resistance device using the property of the magnetic tunnel junction.
Memory cells, each of which contains the MTJ are arranged in a matrix in the MRAM. Of two ferromagnetic layers contained in the MTJ, the direction of magnetization of one ferromagnetic layer (called a pinned ferromagnetic layer) is fixed and the direction of magnetization of the other (called a free ferromagnetic layer) is reversible. Data is stored as the direction of magnetization of the free ferromagnetic layer. A write operation of the data is carried out by supplying a current to the neighborhood of the magnetic tunnel junction to reverse the direction of magnetization of the free ferromagnetic layer by a magnetic field generated by the current. A read operation of the data is carried out by detecting the direction of the magnetization of the free ferromagnetic layer by utilizing the TMR effect.
In the MRAM, it is demanded to realize the write operation of the data (i.e., the reversal of the direction of magnetization) in a small current. On the other hand, in order to hold the data stably, the direction of magnetization of the free ferromagnetic layer is demanded to be stable against thermal disturbance. However, they are generally discrepant from each other. For example, when the coercive force of the free ferromagnetic layer is made small, that is, when an anisotropic magnetic field of the free ferromagnetic layer is made small, the direction of the magnetization can be reversed in a small current. However, the decrease of the anisotropic magnetic field is generally accompanied by the reduction of an energy barrier to prevent the reversal of the direction of magnetization of the free ferromagnetic layer. Therefore, when the anisotropic magnetic field decreases, the MRAM cannot hold the data stably.
U.S. Pat. No. 6,396,735 discloses an MRAM in which the reduction of a write current and the stabilization of data holding are realized at a time. This conventional MRAM contains a magnetic memory device 101a as shown in FIG. 1. The magnetic memory device 101a is composed of a wiring layer 128, an insulating layer 127, an anti-ferromagnetic layer 111, a pinned ferromagnetic layer 112, an insulating layer 113, a free ferromagnetic layer 114, a wiring layer 115 and a ferromagnetic layer 116. The pinned ferromagnetic layer 112 is composed of ferromagnetic layers 120 and 122 and a metal layer 121 put between them. As shown in FIG. 2, the wiring layer 115 extends in parallel to the substrate.
In the MRAM shown in FIG. 1, a write operation of data is carried out by supplying a write current to the wiring layer 115 into a direction parallel to the substrate. When the write current flows through the wiring layer 115, a magnetic field is applied to the free ferromagnetic layer 114 and the ferromagnetic layer 116 so that the directions of the magnetizations of the free ferromagnetic layer 114 and ferromagnetic layer 116 are reversed. Because the direction of the magnetic field applied to the free ferromagnetic layer 114 is opposite to the direction of the magnetic field applied to the ferromagnetic layer 116, the direction of magnetization of the free ferromagnetic layer 114 is opposite to the direction of magnetization of the ferromagnetic layer 116. Because the distance between the wiring layer 115 through which the write current flows and the free ferromagnetic layer 114 or the ferromagnetic layer 116 is short, it is possible to reverse the direction of magnetization in a little current. On the other hand, because the directions of the magnetizations are opposite to each other when the write current does not flow so that the free ferromagnetic layer 114 and the ferromagnetic layer 116 are combined magnetrostatically. Therefore, the directions of the magnetizations of the free ferromagnetic layer 114 and ferromagnetic layer 116 are stable against external disturbance. The technique similar to the technique disclosed in U.S. Pat. No. 6,396,735 is disclosed in U.S. Pat. No. 6,252,796.
In order to stabilize the directions of magnetizations of the free ferromagnetic layer 114 and ferromagnetic layer 116 when the write operation is not carried out, it is preferable that the distance between the free ferromagnetic layer 114 and the ferromagnetic layer 116 is short. The distance between the free ferromagnetic layer 114 and the ferromagnetic layer 116 can be made small by thinning the wiring layer 115. However, when the wiring layer 115 is thinned, the resistance of a path through which the write current flows increases. The increase of the resistance of the write current path is not preferable because it increases power consumption and a delay time.
It is demanded that it is possible to decrease the resistance of the write current path while the direction of the magnetization of the free ferromagnetic layer is further stabilized in case of a non-write operation.
In conjunction with the above description, a magnetic tunnel junction device is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 11-161919). The magnetic tunnel junction device of this conventional example contains a pinned ferromagnetic multiple layer, a free ferromagnetic multiple layer and an insulation tunnel layer. The pinned ferromagnetic multiple layer has a magnetic moment fixed in a desired direction under application of a magnetic field, and contains first and second ferromagnetic films which are combined anti-ferromagnetically with each other, and an anti-ferromagnetic coupling film arranged to contact the first and second ferromagnetic films. The free ferromagnetic multiple layer has a freely reversible magnetic moment under application of a magnetic field and contains first and second ferromagnetic films which are combined anti-ferromagnetically with each other, and an anti-ferromagnetic coupling film arranged to contact the first and second ferromagnetic films. The insulation tunnel layer is arranged to contact the pinned ferromagnetic multiple layer and the free ferromagnetic multiple layer to permit a tunnel current between the pinned ferromagnetic multiple layer and of the free ferromagnetic multiple layer.
Also, a magneto-resistance effect device is disclosed in Japanese Laid Open Patent Application (JP-P2002-141583A). The magneto-resistance effect device of this conventional example is composed of a free layer, a non-magnetic layer, and a pinned layer. A current flows between the free layer and the pinned layer through the non-magnetic layer, and a vertical bias layer is provided to contact the free layer. The vertical bias layer is formed for the lower surface of the free layer to have a plane contact with the upper surface of the vertical bias layer or for the upper surface of the free layer to have a plane contact with the lower surface of the vertical bias layer.